1. Field of the Invention
The present invention relates generally to exothermic materials and, more particularly, is concerned with an ignitor having a stable low-energy thermite igniting system for exothermic materials which are difficult to initiate, such as consolidated thermite and other comparable materials.
2. Description of the Prior Art
Thermite is a mixture of powdered iron oxide, Fe.sub.2 O.sub.3, and powdered or granular aluminum, Al, or any other metal/metal oxide mixture that reacts exothermically. When it is heated, the following typical reaction occurs:
Fe.sub.2 O.sub.3 +2Al.fwdarw.Al.sub.2 O.sub.3 +2Fe+185,000 calories.
This reaction, sometimes called the Goldschmidt reaction, is sufficiently exothermic that maximum temperatures can exceed 4000 degrees F. The iron formed melts and easily ignites any combustible material contacted. Consequently, the reaction is self-sustaining.
The energy requirements to ignite materials such as consolidated thermite are very high, making initiation of the thermite reaction difficult. Traditionally, either a readily ignited chemical heat source or an electrical power source in excess of 100 watts has been used. In an effort to reduce electrical power requirements, "staged" pyrotechnic devices have been devised. These devices use sensitive pyrotechnic mixtures next to an electrical ignition device, commonly a bridgewire, which is the electrical input medium. The drawback with this arrangement is that one or more multi-component pyrotechnic mixtures are required to reach the desired thermal output to ignite the consolidated thermite. The use of several pyrotechnic mixtures raises the issue of compatibility of the chemical entities involved. The use of sensitive materials increases the likelihood that corrosion or desensitizing reactions may occur during storage, making the ignitor useless.
A few representative examples of prior art ignitors are those disclosed in U.S. Pat. Nos. to Zeman (3,134,329), Colburn, Jr. et al (3,160,097), Martin (3,732,129) and Cannavo et al (4,354,432). The devices disclosed by Zeman, Colburn, Jr. et al and Martin are all designed to operate in the exploding bridgewire mode and use secondary high explosives or classical pyrotechnic or propellant mixtures on the bridgewire. The device disclosed by Cannavo et al is designed to operate in a hot-wire mode and contains classical pyrotechnics of many components and binders. In particular, different three-part chemical compositions are used for the "initiator" and "ignitor" portions of the device. The initiator portion is zirconium/lead chromate/silicone resin and the ignitor portion is aluminum/cuprous oxide/nitro polymer.
In view of the aforementioned shortcomings and drawbacks of prior art ignitors, a need still exists for an ignitor of simple construction suitable for igniting materials which are difficult to initiate, such as consolidated thermite and other comparable materials.